Wideband oscillators are well known in a variety of electronic arts and often include means to effect electronic tuning. That is, such oscillators often include a circuit element that can be controlled, by application of an electronic frequency control signal, to vary the oscillator's operating frequency.
There are a number of operational parameters that can be of concern in oscillator operation, depending on the oscillator's intended application. In oscillators intended for measurement instrumentation applications, for example, "phase noise" is often a critical parameter. Other parameters of concern may include output power and compression angle. Many of these parameters are strongly influenced by the bias conditions (such the voltage applied to a collector of an oscillator transistor) under which the oscillator is operated.
In an oscillator intended for fixed frequency operation, it is relatively straightforward to select the bias conditions so that the parameters of particular concern are optimized. However, a serious problem is evident in oscillators that can be tuned to operated over a broad band of frequencies: what bias conditions should be selected?
The usual approach is to select fixed, compromise bias conditions that permit adequate (rather than optimum) oscillator operation over a desired range of frequencies. By so doing, however, optimum oscillator performance is achieved at only one frequency, if at all. Further, the use of fixed bias conditions necessarily limits the range of possible operating frequencies, sometimes preventing certain design criteria from being met.
An alternative approach has been to try and design the oscillator circuit so that the optimum bias voltage changes, as a function of frequency, in exactly the same manner and magnitude as the frequency control signal changes as a function of frequency. In such cases, the same signal is used both for the frequency control signal and the bias signal. However, the fortuitous circumstances that permit such dual use of a single signal are quite rare, and the utility of this approach is thus quite limited.
Another approach sometimes used is to electronically change the configuration of the oscillator circuit as the tuning voltage is changed. This can be accomplished by means such as PIN diodes that switch certain circuit elements into or out of the circuit when predetermined tune voltage thresholds are passed. This approach, however, suffers by reason of its complexity and by reason of parasitic problems associated with the additional circuit elements.
The problem of achieving optimum oscillator bias is compounded by the fact that the optimum bias point changes with changes in temperature. An oscillator with fixed bias may thus fail to oscillate at certain frequencies if the ambient temperature is too high or low.
While compensation against changes in oscillator frequency due to temperature changes is known, no one has heretofore compensated against changes in optimum oscillator bias due to temperature changes.
While it might be proposed to extend the above-described "dual use of a single signal" compensation technique to encompass temperature compensation too, such an approach would be unavailing. Any adjustment to the oscillator bias to account for temperature effects would necessarily be accompanied by an unwanted change in the oscillator frequency.
In essence, then, wideband oscillators typically include compound compromises: compromises of oscillator bias as a function of frequency, and compromises of oscillator bias as a function of temperature. The result is an oscillator that performs poorly under most of its operating conditions.
In accordance with the present invention, these drawbacks of the prior art are addressed by circuitry that adjusts an oscillator's bias in response to changes in oscillator frequency and/or ambient temperature, and does so without resort to using the same signal for both bias and frequency control. By so doing, both the frequency range and temperature range of an oscillator can be extended, while simultaneously improving the oscillator's performance.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.